After Koyomi Araragi catches his classmate, Hitagi Senjougahara, when she fell down some stairs, he makes a strange discovery about her. Trying to keep her secret Hitagi attacks Koyomi. However, he has a secret of his own and might be able to help her.

To overcome her problems, Hitagi has to face some difficult and painful memories from her past.

Koyomi Araragi has an argument with his sister and then finds it difficult to stay in the house. He goes to the park where he meets Hitagi Senjyogahara. They see a little girl, Mayoi Hachikuji, comparing the town map displayed in the park to the memo in her hand. Learning that Mayoi has lost her way and route to a relative's home, Koyomi offers to help, only to be violently rebuffed.

After Koyomi Araragi and Hitagi Senjyogahara discover that the girl they met in the park, Mayoi Hachikuji, is trying to get to her mother's house, they offer to walk along with her. However, they keep getting incredibly lost.

While Hitagi is off to Oshino for advice, Koyomi learns more about Mayoi's life. When Hitagi finally returns, she reveals some information that surprises Koyomi.

On his way to Hitagi's house, Koyomi bumps in to Suruga Kanbaru, the school's basketball star and an old friend of Hitagi, of whom he can't get rid of. Later that day, Koyomi is attacked by a mysterious being.

The day after his attack, Koyomi goes to see Suruga, where she tells him something about herself he can't quite grasp.

Koyomi takes Suruga to see Oshino in order to help her. However, the help he offers isn't the help Koyomi wanted...

Koyomi and Suruga travel to an abandoned shrine in order to perform a favor for Oshino. As they climb the mountain the shrine is located on, they pass a girl running down the path, who only briefly exchanges glances before continuing in the other direction. Once they reach the shrine, Koyomi and Suruga discover a number of dismembered snake corpses in what appears to be some sort of ritual. They both quickly decide to leave. Later, Koyomi comes across Hanekawa in a bookstore, where they begin to converse. Hanekawa warns Koyomi that he and Hitagi must set down boundaries in their relationship if it is to last. She then leaves after getting a sudden headache. Koyomi then meets the girl again, who he finally realizes is Nadeko Sengoku, a friend of his sisters who he met as a child. He and Suruga intercept her at the shrine, and return to Koyomi's house. Once there, Nadeko takes off her clothes, revealing snakelike skin, and pleads for Koyomi to save her.

Oshino informs Koyomi that Nadeko is under the effects of a Snake Constrictor, a snake spirit that slowly constricts the life out of its victim. Nadeko explains that it is a curse a classmate placed on her after Nadeko inadvertently rejected the confession of a boy the classmate liked. Normally, a curse placed by an amateur would be harmless, but Nadeko accidentally made the curse worse by performing her ritual at the shrine, which has been corrupted by Shinobu's presence. Koyomi obtains a special charm that will banish the Snake Constrictor and they return to the shrine. While the ritual is seemingly successful, Koyomi realizes that Nadeko is under the attack of two Snake Constrictors, the second being placed by the rejected boy. While the first one is banished, Koyomi is forced to battle the second to keep it from killing Nadeko. He and Kanbaru manage to chase it off, but Koyomi feels guilty for not being able to destroy it, as the Snake Constrictor will return to the person who originally placed the curse.

After school Koyomi finds Nadeko and they talk about how Shinobu looked at her and Suruga with a different gaze than Koyomi and Oshino. Tsubasa appears and Nadeko leaves abruptly, Koyomi and Tsubasa talk about how Koyomi has become a bad influence to Hitagi in school. Tsubasa gets a headache and Koyomi flashbacks into what happened in Golden Week, Tsubasa turned into a white "cat-woman" and attacked people by absorbing their vitality. Oshino reveals the problem is because of a huge amount of stress built up over the years. Koyomi acts as bait to attract the cat and Shinobu bites her draining her energy until she turns back into Tsubasa.

Hitagi and Koyomi are having lunch. Her teases him and roughly "asks" him to go on a date. He comes late but Hitagi doesn't seem to mind. Her father drives them to a location where Hitagi teases him more. She leaves him with her father where at first Koyomi was nervous. Her father goes on talking about how he isn't there for her often and asks Koyomi to take cake of her. Hitagi blindfolds him and then shows him an area where a blanket is laid on the ground. They lay down and she tells him that the stars are all she has rest to give him. She tells him that she loves him and Koyomi returns the same feelings. She asks him to kiss her.